The present invention relates to a hybrid riser for deep water.
Several configurations for transporting the fluid from the field between a well head and/or manifold and surface support have been proposed.
The configurations used depend, in general, on the exploitation site, the parameters relating, in particular, to the depth of water and the horizontal and vertical movements of the surface support being taken into consideration in order to select the appropriate configuration and/or the type of riser.
A first known configuration is the configuration known as the free-hanging configuration. In this configuration, the riser has, on the one hand, an upper part which may be considered as being vertical because it makes an angle of between 5 and 30xc2x0 with respect to a vertical, and, on the other hand, a lower part, one portion of which is curved and one portion of which is horizontal resting on the sea bed. The transition between the curved and horizontal portions occurs at the region where the pipe touches down on the sea bed. In this configuration, and regardless of the type of riser used, the compensation for the vertical movement which causes a heave effect occurs in the lower part of the riser, that is to say in the touch-down region. This heave leads to significant fatigue in the curved portion of the riser.
When a riser, in this free-hanging configuration, consists of a rigid tube or of two concentric rigid tubes, it is known as a steel catenary riser or SCR; the radius of curvature of the curved portion which must not exceed the yield strength of the metallic material of which the SCR is made is relatively large, of the order of 100 meters and even more. Such a large radius of curvature has the effect of moving the point of touch-down away from the vertical line passing through the point of connection to the surface support, and this restricts its use in production zones comprising several wells because one SCR would, in theory be needed for each well. Furthermore, an SCR tolerates very little vertical variation to compensate for the vertical movement of the surface support. In general, the vertical variation has to be less than 1 to 2% of the water""s depth. Likewise, the horizontal displacements are limited because they introduce additional fatigue into the curved portion. Finally, the SCR requires the use of a rotary joint at the top where it connects to the surface support.
In spite of its advantages in relation to good thermal insulation and an acceptable cost per unit length, it is nonetheless true that it is little used when the same item of surface support is to serve several well heads and/or when the displacements of the surface support are great compared with the depth of water.
A flexible pipe may be used in deep seas in the free-hanging configuration. It has advantages over the SCR, for example, a far smaller radius of curvature at the curved portion meeting the sea bed, the said radius of curvature having to be greater than the MBR (Minimum Bend Radius) and is typically of the order of 2 to 15 m, namely at least ten times smaller than in the case of the SCR. Furthermore, it allows greater vertical and horizontal movements of the surface support thanks to its better fatigue behaviour. However, it has the drawbacks of being very heavy, not having such good thermal insulation as the SCR, and having a higher cost per unit length than the SCR.
Thus, the free-hanging configuration may be used in seas which are not too deep with surface support that is mobile or tethered to the sea bed and, in general, in sites in which there are few waves and marine currents. For deep seas, the risers are very heavy and, in order to avoid very large suspended weights, it is preferable to use other configurations.
One other configuration consists in appropriately mounting buoyancy means with positive buoyancy so as to distribute the suspended weight between the upper and lower parts of the riser. The riser may consist either of a tower or of an SCR combined with another element which may be laid in various configurations known as LAZY or STEEP S or WAVE, these configurations being represented in the documents API 17B and 17J (AMERICAN PETROLEUM INSTITUTE).
A tower is mounted vertically from the sea bed to which it is fixed by appropriate means up to a certain distance from the surface support. Because of the very substantial weight of the tower, which internally contains a certain number of risers, enormous buoyancy means are intended to be installed at the top to take up most of the weight of the tower. Furthermore, flexible lines with their concave side facing upwards connect the tower to the surface support. These lines, which may consist of short lengths of flexible pipe known as jumpers, are intended, among other things, to allow relative movement between the surface support and the tower. The bundle of flow lines incorporated into the tower may comprise one or more gas lift lines. Furthermore, heave compensation may be achieved by rams mounted between the surface support and the top of the tower.
Another hybrid configuration uses a riser in which the lower part consists of an SCR and the upper part consists of a short flexible pipe (jumper) arranged on buoyancy means such as a buoy known as a buoyancy arch. Thus, the weight of the riser is taken up by the buoyancy means and heave is compensated for by the short length of flexible pipe. However, laying risers in all configurations using mid-water buoyancy means is a relatively lengthy and difficult task. This is because the mid-water buoyancy means are fitted before the riser is laid. These buoyancy means are connected by tethers to deadweights or anchors set in the sea bed. The SCR is then laid, connecting it to the buoyancy means. The short flexible pipe (or jumper) is connected to the floating structure.
Another drawback is associated with the total length of riser which is longer than it would be in a free-hanging configuration.
As to the LAZY or STEEP S or WAVE configurations employing a completely flexible pipe, it is essential that buoyancy means with positive buoyancy be associated with part of the flexible pipe. In the case of an arch, the deadweight has to be lowered onto and secured to the sea bed at a very precise determined point. Then the arch has to be fixed to the deadweight by tethers or chains before the laying of the riser can be resumed. In the case of buoyancy means consisting of buoys arranged in a string along part of the pipe, it is necessary to interrupt the laying of the flexible pipe in order to attach each buoy to the part of the pipe. That operation being is performed on the deck of the laying vessel.
Thus, whatever the envisaged configuration and type of riser used, additional means are needed to take up part of the weight of the riser and/or special means are required to reduce the effect of the heave in the region of touchdown on the sea bed, while keeping in mind the fact that the higher the weight of the rigid part of the riser, when integrated, the larger the buoyancy means will be, which gives rise to a greater drag force (or drag diameter) and greater hydrodynamic loadings.
The object of the present invention is to propose a new hybrid configuration which makes it possible to eliminate the use of auxiliary techniques such as buoyancy means, the tensioning cables, the flexible or rotating joints and anchorage to the sea bed.
The present invention concerns a hybrid riser, which it comprises a metallic rigid central part, having an upper end which is connected to at least one upper portion of flexible pipe of predetermined length and another lower end which is connected to a portion of flexible pipe of a length at least equal to the length of the upper portion of flexible pipe.
According to another feature of the present invention, the rigid central part is connected to the upper portion of flexible pipe and to the lower flexible pipe by respective fixed coupling devices.
One advantage of the present invention is that the riser obtained is rigid for the most part and has the properties of flexible pipes where it leads off from the surface support and where it touches down on the sea bed.
In addition, the dynamic stresses to which the hybrid riser may be subjected are easily absorbed or compensated for both in the upper part and in the lower part of the riser, by virtue of the presence of true flexible pipes, which allows large excursions of the surface support from which the riser is suspended and allows these without overbending.
Finally, because it is no longer necessary to use buoyancy means, the riser according to the invention can be installed in the manner of a free-hanging configuration. Thus, the riser is as short as possible and has a mean cost per unit length which is roughly equivalent to that of an SCR while at the same time employing flexible portions.
Furthermore, the upper flexible portion takes up all of the weight of the central rigid part and of the lower flexible portion, whereas the latter takes up practically all the dynamic stresses generated by the movements of the surface support. Thus, the surface support can move horizontally and vertically over relatively large distances without significant effects on the riser according to the present invention because the riser is capable of accompanying the movements of the surface support and of doing so without generating additional fatigue and/or additional wear in the curved part of the lower portion which, by virtue of its flexibility, is able to move along the sea bed and appropriately absorb the heave effect.
According to the present invention, the lower portion of flexible pipe includes at least one section which is in the shape of a wave and which is formed between the end for a connection to the rigid central portion and the touch down point with the sea bed.
One advantage of the invention lies in reducing the angle at the top of the riser to bring it down about 3xc2x0, which enables the amplitude of the movements of the said riser under severe conditions of use to be increased.